Field of the Application
The application generally relates to Circuit Switch Fallback (CSFB), and more particularly, to apparatuses and methods for reducing CSFB call setup time.
Description of the Related Art
With growing demand for ubiquitous computing and networking, various Radio Access Technologies (RATs) have been developed, including the Global System for Mobile communications (GSM) technology, General Packet Radio Service (GPRS) technology, Enhanced Data rates for Global Evolution (EDGE) technology, Wideband Code Division Multiple Access (WCDMA) technology, Code Division Multiple Access 2000 (CDMA2000) 1× technology, Time Division-Synchronous Code Division Multiple Access (TD-SCDMA) technology, Worldwide Interoperability for Microwave Access (WiMAX) technology, Long Term Evolution (LTE) technology, Time-Division LTE (TD-LTE) technology, and LTE-Advanced (LTE-A) technology, etc.
To provide the user with convenience and flexibility, most User Equipment (UE) nowadays supports at least two RATs, and as such may be referred to as a multiple-RAT UE. Using a UE that supports both the WCDMA technology and the LTE technology as an example, it may selectively obtain wireless services using the WCDMA technology or the LTE technology. Generally, it selects an LTE network over a WCDMA network when wireless services are available from both service networks, since the LTE network may be more likely to provide wireless services with higher data throughput than the WCDMA network. Nonetheless, in some cases, a technique called Circuit-Switched Fallback (CSFB) may be employed for a multiple-RAT UE which is already camped on an LTE network to switch to a WCDMA network for accessing, particularly, Circuit-Switched (CS) services, e.g., a CS call. A CS call made utilizing the CSFB technique is generally referred to as a CSFB call.
According to the 3rd Generation Partnership Project (3GPP) Technical Specification (TS) 24.301, v13.1.0, when a multiple-RAT UE camped on an LTE network receives a TRACKING AREA UPDATE REJECT message with a certain rejection cause during the combined Tracking Area Updating (TAU) procedure, the multiple-RAT UE should perform the attach procedure with the LTE network. In the worst case, the multiple-RAT UE may have to retry the attach procedure up to four times if the attach procedure is not successfully completed, and there is a guard period to wait before each retry of the attach procedure. That is, the multiple-RAT UE may spend a long time retrying the attach procedure with the LTE network. However, there may be situations where a request for a CSFB call is received during the combined TAU procedure but before the TRACKING AREA UPDATE REJECT message. As a result, the processing of the request for the CSFB call may be seriously delayed due to the unsuccessful attach procedure.